The Advances, Challenges, and Perspectives on Electrocatalytic Reduction of Nitrogenous Substances to Ammonia: A Review

Ammonia (NH3) is considered to be a critical chemical feedstock in agriculture, industry, and other fields. However, conventional Haber–Bosch (HB) ammonia (NH3) production suffers from high energy consumption, harsh reaction conditions, and large carbon dioxide emissions. Despite the emergence of electrocatalytic reduction of nitrogenous substances to NH3 under ambient conditions as a new frontier, there are several bottleneck problems that impede the commercialization process. These include low catalytic efficiency, competition with the hydrogen evolution reaction, and difficulties in breaking the N≡N triple bond. In this review, we explore the recent advances in electrocatalytic NH3 synthesis, using nitrogen and nitrate as reactants. We focus on the contribution of the catalyst design, specifically based on molecular–catalyst interaction mechanisms, as well as chemical bond breaking and directional coupling mechanisms, to address the aforementioned problems during electrocatalytic NH3 synthesis. Finally, we discuss the relevant opportunities and challenges in this field.


Introduction
With the continuous development of industry and agriculture, the yearly increase in carbon dioxide (CO 2 ) emissions has resulted in numerous environmental problems and has drawn extensive attention from the international community.Therefore, reducing the carbon footprint by optimizing chemical processes is of paramount importance [1].Ammonia (NH 3 ) is not only one of the most productive industrial chemicals worldwide, but also serves as the cornerstone of modern industrial and agricultural development [2,3].For instance, NH 3 can be used to manufacture various products, including fertilizers, explosives, plastics, synthetic fibers, and dietary proteins, among others, which dominate industrial applications [2,4].However, conventional Haber-Bosch (HB) ammonia (NH 3 ) production still faces significant challenges.The Haber-Bosch process involves reacting hydrogen and nitrogen over an iron-based catalyst at temperatures approaching 500 • C and pressures up to 300 bar to form ammonia [2].On one hand, this process leads to the release of a large amount of carbon compounds, such as CO and CO 2 , during the production process, contributing to the severe climate crisis and extreme weather conditions (e.g., melting Arctic permafrost, large-scale coral bleaching, etc.) [5][6][7][8][9].On the other hand, the smooth operation of this process heavily depends on massive fossil fuel combustion and substantial energy consumption.In reality, thermal-catalytic NH 3 synthesis technology requires harsh reaction conditions (Figure 2).To overcome this dilemma, innovative strategies such as photocatalytic, electrocatalytic, and chemical looping approaches have been proposed for effective NH 3 synthesis [10][11][12][13][14][15].requires harsh reaction conditions (Figure 2).To overcome this dilemma, innovative strategies such as photocatalytic, electrocatalytic, and chemical looping approaches have been proposed for effective NH3 synthesis [10][11][12][13][14][15].N2 and NO3 − are currently regarded as the most extensively used feedstocks for electrocatalytic NH3 synthesis.Although this reaction possesses excellent characteristics, there are several challenges in actual experiments, including the difficulty of N2 adsorption and activation, as well as the difficulty of directional coupling of products.However, through relentless efforts, scientists have identified solutions to these problems based on two mechanisms: the molecular-catalyst interaction, and chemical bond breaking and directional coupling.The molecular-catalyst interaction mechanism refers to the process in which a catalyst is added to the reaction system and interacts with the reactants to activate them or modify the reaction rate.The mechanism of chemical bond breaking and directional coupling, on the other hand, involves breaking the chemical bond of a reactant un- requires harsh reaction conditions (Figure 2).To overcome this dilemma, innovative strategies such as photocatalytic, electrocatalytic, and chemical looping approaches have been proposed for effective NH3 synthesis [10][11][12][13][14][15].N2 and NO3 − are currently regarded as the most extensively used feedstocks for electrocatalytic NH3 synthesis.Although this reaction possesses excellent characteristics, there are several challenges in actual experiments, including the difficulty of N2 adsorption and activation, as well as the difficulty of directional coupling of products.However, through relentless efforts, scientists have identified solutions to these problems based on two mechanisms: the molecular-catalyst interaction, and chemical bond breaking and directional coupling.The molecular-catalyst interaction mechanism refers to the process in which a catalyst is added to the reaction system and interacts with the reactants to activate them or modify the reaction rate.The mechanism of chemical bond breaking and directional coupling, on the other hand, involves breaking the chemical bond of a reactant un-  − are currently regarded as the most extensively used feedstocks for electrocatalytic NH 3 synthesis.Although this reaction possesses excellent characteristics, there are several challenges in actual experiments, including the difficulty of N 2 adsorption and activation, as well as the difficulty of directional coupling of products.However, through relentless efforts, scientists have identified solutions to these problems based on two mechanisms: the molecular-catalyst interaction, and chemical bond breaking and directional coupling.The molecular-catalyst interaction mechanism refers to the process in which a catalyst is added to the reaction system and interacts with the reactants to activate them or modify the reaction rate.The mechanism of chemical bond breaking and directional coupling, on the other hand, involves breaking the chemical bond of a reactant under certain conditions and forming a new chemical bond after a specific treatment to synthesize a desired substance.This review summarizes the current research status of electrocatalytic ammonia synthesis using nitrogen and nitrate, focusing on the two mechanisms of molecular-catalyst interaction and chemical bond breaking and directional coupling.Additionally, it presents new challenges in the electrocatalytic ammonia synthesis process.During electrocatalytic ammonia synthesis using nitrogen as the raw material, there is competition between the N 2 reduction reaction (NRR) and the hydrogen evolution reaction (HER), as they have similar overpotentials.This competition results in a low Faraday efficiency (FE) of the reaction, leading to significant wastage of the raw materials and increased energy consumption [16][17][18].To address this issue, researchers have made continuous efforts to find new catalysts that improve the selectivity of the main reaction through the mechanism of the molecular-catalyst interaction.However, experiments have shown that the use of classical iron catalysts leads to significant energy loss, necessitating modifications to the existing catalysts.

Inhibiting the Competition from the Hydrogen Evolution Reaction
Electrocatalytic systems, which utilize water as the hydrogen source instead of fossil fuels, often rely on renewable energy sources, such as solar, wind, and hydro power, to provide the necessary energy for the NRR (Figure 3) [19].However, one of the main challenges in the NRR is that the HER is kinetically more favorable than the NRR, which negatively impacts the efficiency of the NRR and reduces ammonia production.Therefore, it is crucial to screen and design electrocatalysts with high catalytic activity and selectivity.Significant progress has already been achieved in this field [20].
der certain conditions and forming a new chemical bond after a specific treatment to synthesize a desired substance.This review summarizes the current research status of electrocatalytic ammonia synthesis using nitrogen and nitrate, focusing on the two mechanisms of molecular-catalyst interaction and chemical bond breaking and directional coupling.Additionally, it presents new challenges in the electrocatalytic ammonia synthesis process.

Mechanism of Molecular-Catalyst Interaction for Electrocatalytic N2 Reduction Reaction
During electrocatalytic ammonia synthesis using nitrogen as the raw material, there is competition between the N2 reduction reaction (NRR) and the hydrogen evolution reaction (HER), as they have similar overpotentials.This competition results in a low Faraday efficiency (FE) of the reaction, leading to significant wastage of the raw materials and increased energy consumption [16][17][18].To address this issue, researchers have made continuous efforts to find new catalysts that improve the selectivity of the main reaction through the mechanism of the molecular-catalyst interaction.However, experiments have shown that the use of classical iron catalysts leads to significant energy loss, necessitating modifications to the existing catalysts.

Inhibiting the Competition from the Hydrogen Evolution Reaction
Electrocatalytic systems, which utilize water as the hydrogen source instead of fossil fuels, often rely on renewable energy sources, such as solar, wind, and hydro power, to provide the necessary energy for the NRR (Figure 3) [19].However, one of the main challenges in the NRR is that the HER is kinetically more favorable than the NRR, which negatively impacts the efficiency of the NRR and reduces ammonia production.Therefore, it is crucial to screen and design electrocatalysts with high catalytic activity and selectivity.Significant progress has already been achieved in this field [20].Two types of catalysts, metallic and non-metallic, are currently the most extensively studied [16,21].Metallic catalysts can be categorized as precious metal and non-precious metal catalysts.Electrochemical ammonia synthesis using precious metal catalysts is highly sensitive to the catalytic structure.The energy barrier for nitrogen dissociation depends largely on the particle size and crystal structure of the metal active center.For the NRR, a simple and effective method of electroplating rhodium and ruthenium on titanium Two types of catalysts, metallic and non-metallic, are currently the most extensively studied [16,21].Metallic catalysts can be categorized as precious metal and non-precious metal catalysts.Electrochemical ammonia synthesis using precious metal catalysts is highly sensitive to the catalytic structure.The energy barrier for nitrogen dissociation depends largely on the particle size and crystal structure of the metal active center.For the NRR, a simple and effective method of electroplating rhodium and ruthenium on titanium felt using an electrochemical membrane reactor has been proposed.Linear sweep voltammetry confirms that Ru and Rh coatings can promote the electrochemical synthesis of NH 3 .Among the metal catalysts, Ru is particularly suitable for electrochemical NH 3 synthesis due to its higher activity [22,23].Additionally, a model combining a porous graphite carbon nitride matrix (Ru SAs/g-C 3 N 4 ) has been suggested [24], which allows control over the reaction selectivity by adding an applied potential, thereby adjusting the local chemical environment and size of the active site of Ru.The highly dispersed Ru sites on the g-C 3 N 4 substrate significantly enhanced the exposure of the catalytic active site and the emerged concerted interplay between the single-atom sites and the substrate regulating the electron configuration of the metal sites, which prompted the adsorption of the key intermediates and, thus, achieved a superior NRR performance.Due to the extremely low catalytic activity (typically in the order of micrograms), traces of NO x pollutants in the atmosphere may cause false-positive results in the above reactions.Therefore, for Ncontaining catalysts, it is critical to eliminate possible interference with the reaction results from airborne contaminants and the catalyst itself.The authors used 15 N 2 isotope labelling experiments to demonstrate that the N in NH 3 indeed originates from N 2 and not from other pollutants.As electrocatalytic NRR has received more attention from researchers, it is particularly important to establish a rigorous reaction process to exclude interference from airborne pollutants and the catalyst itself.As delineated in Scheme 1, each separate component of the electrolyte solution (e.g., electrolyte salt, solvent, and electrolyte solution) should be confirmed that it is not polluted by other forms of contamination.Besides, all the glassware, electrodes, and labware were kept inside a sealed electrochemical cell, with no potential applied for 24 h to exclude possible contamination processes.Rigorous gas purification processes and electrocatalytic tests (including the setting up of multiple comparative and repetitive experiments, as well as the cross-checking of product yields using different test methods) are indispensable to ensure the rigor of the electrocatalytic nitrogen reduction reaction.Monatomic catalysts, although limited in the size of the reduction, offer nearly 100% metal dispersion, and they are usually the active sites for the electrochemical reduction of nitrogen and are able to form stable chemical bonds with nitrogen atoms, which facilitates the breaking of the N≡N bond [25].It enables them to exhibit good activity and selectivity.However, the scarcity and high cost of precious metal catalyst materials hinders their large-scale industrial application.To overcome this limitation, it is important to explore low cost and abundant alternative materials, such as non-precious metals, and conduct research to enhance their electrocatalytic performance as potential substitutes for precious metal catalysts.For example, Mo has been widely studied as a non-noble metal catalyst in the electrochemical synthesis of ammonia due to its excellent electrocatalytic performance [22,24].Non-precious metal catalysts, while overcoming the high-cost challenge, face the problem of low FE [26].Metal catalysts are d-orbital catalysts, whereas non-metal catalysts activate inert N 2 molecules through sp hybrid orbitals, and this difference in the activation mechanism also renders the nonprecious metal NRR catalysts more suitable for the N 2 molecules.The different activation mechanisms also make the non-metallic catalysts valuable for research.
In recent decades, there has been a proliferation of metal-free catalysts [27].Researchers quickly realized the potential benefits of these catalysts and have begun making various modifications to make them more widely used in place of traditional metal-based catalysts.Based on the characteristics of the NRR, it is crucial to find metal-free catalysts with abundant active sites to enhance the adsorption and activation of N 2 .Carbon-based materials doped with N and P have shown promise as highly active materials suitable for NRR due to their excellent electrical conductivity and durability [28].The surface of defective carbon materials provides unsaturated coordination sites for the chemisorption of N 2 .For example, in 2018, scientists combined a metal-free polymer, carbon nitride, with abundant Nvs.(PCN-NVs) to act as a highly active nitrogen reduction reaction (NRR) catalyst in order to significantly enhance the ammonia (NH 3 ) yield and FE.The PCN-Nvs.demonstrated excellent N 2 adsorption and activation capabilities due to the interconnectedness of the materials, resulting in exceptional properties.The experimenters used isotopic labeling in this experiment to demonstrate that all the NH 3 detected in the results came from the supplied N 2 , eliminating all possible contamination from the N-containing catalyst and ensuring the accuracy of the results [16,29].Scheme 1.The NH3 electrosynthesis experimental procedures utilized for excluding potential contaminants.
Scheme 1.The NH 3 electrosynthesis experimental procedures utilized for excluding potential contaminants.
Over time, considering the good performance of Ni-based catalysts for NRR, researchers began to explore the design and synthesis of Ni-based catalysts with certain exposed surfaces, such as nickel telluride (NiTe) nanocrystals [30,31].By employing a straightforward synthesis process, scientists were able to selectively expose the {001} and {010} surfaces of NiTe nanocrystals.This precise atomic-level manipulation of the surface chemistry allowed for high selectivity in the NRR process.Remarkably, the scientists successfully designed the surface atomic structure of NiTe nanocrystals to achieve efficient NRR at 0.1 µm HCl.The results demonstrated that NiTe nanocrystals with {001} surface exposure exhibited superior NRR performance compared to those with {010} surface exposure (Figure 4).This enhanced activity can be primarily attributed to the optimized surface atomic arrangement of the exposed {001} surface, which allowed for efficient electrical reduction of N 2 to NH 3 through an alternating mechanism.On the contrary, {010} surfaces, consisting of a single Te bit, had a surface structure that obstructed the NRR process, resulting in poor performance.Engineering the surface atomic architecture of the catalyst to selectively expose the {001} facets guaranteed the adsorption and activation of N 2 and weakened the binding for *H (* means catalytically active site) species.This work provides valuable guidance for the expansion and enhancement of catalyst properties [30,31].
NVs (PCN-NVs) to act as a highly active nitrogen reduction reaction (NRR) catalyst in order to significantly enhance the ammonia (NH3) yield and FE.The PCN-NVs demonstrated excellent N2 adsorption and activation capabilities due to the interconnectedness of the materials, resulting in exceptional properties.The experimenters used isotopic labeling in this experiment to demonstrate that all the NH3 detected in the results came from the supplied N2, eliminating all possible contamination from the N-containing catalyst and ensuring the accuracy of the results [16,29].
Over time, considering the good performance of Ni-based catalysts for NRR, researchers began to explore the design and synthesis of Ni-based catalysts with certain exposed surfaces, such as nickel telluride (NiTe) nanocrystals [30,31].By employing a straightforward synthesis process, scientists were able to selectively expose the {001} and {010} surfaces of NiTe nanocrystals.This precise atomic-level manipulation of the surface chemistry allowed for high selectivity in the NRR process.Remarkably, the scientists successfully designed the surface atomic structure of NiTe nanocrystals to achieve efficient NRR at 0.1 µm HCl.The results demonstrated that NiTe nanocrystals with {001} surface exposure exhibited superior NRR performance compared to those with {010} surface exposure (Figure 4).This enhanced activity can be primarily attributed to the optimized surface atomic arrangement of the exposed {001} surface, which allowed for efficient electrical reduction of N2 to NH3 through an alternating mechanism.On the contrary, {010} surfaces, consisting of a single Te bit, had a surface structure that obstructed the NRR process, resulting in poor performance.Engineering the surface atomic architecture of the catalyst to selectively expose the {001} facets guaranteed the adsorption and activation of N2 and weakened the binding for *H (* means catalytically active site) species.This work provides valuable guidance for the expansion and enhancement of catalyst properties [30,31].However, when selecting catalysts to inhibit the hydrogen evolution reaction (HER), it is important to note that uncontrolled HER inhibition may not be a reasonable strategy for developing an effective NRR catalyst.From the point of view of the reaction mechanism, NRR is a process in which N 2 reacts with protons and electrons to form NH 3 .This process can be briefly summarized in three stages: (1) the protons acquire electrons on the catalyst surface, thus H + e − + * = H*; (2) activation of N 2 occurs in which gaseous N 2 reacts with surface H* species to form N 2 H 2 *, thus N 2 (g) + 2H* = N 2 H 2 *; (3) after the formation of N 2 H 2 * species, protons and electrons will keep attacking the N 2 H 2 * group to produce two NH 3 molecules.The reactions in this process are all exothermic, so it is known that N 2 H 2 will not appear as a final product, according to reaction kinetics and thermodynamics [32].In this scenario, HER plays a dual role in the electrocatalytic NRR process.On one hand, the HER competes with the NRR, leading to decreased ammonia yield and FE.On the other hand, excessive HER inhibition can hinder the hydrogenation of nitrogen (N), since an appropriate amount of hydrogen (H) is required in the electrolyte for N 2 protonation.In theory, if a catalyst cannot activate protons, it will struggle to hydrogenate N≡N.Molybdenum (Mo), cobalt (Co), and platinum (Pt)-based materials are known to exhibit HER activity.Recent studies have shown that utilizing these materials as catalysts for NRR has achieved relatively high NRR performance, surpassing the abovementioned catalysts that simultaneously inhibit the HER and promote the NRR [33,34].

Retrofit Existing Catalysts to Reduce Energy Consumption
Most commercial plants currently employ multi-catalyzed iron-based catalysts; however, this process requires high temperatures and pressures, leading to significant energy consumption.In light of this, numerous researchers have reported novel active catalysts for ammonia synthesis under milder conditions.Following the groundbreaking report by Aika et al. on the highly active Ru catalyst for ammonia synthesis, the scientific community has directed its attention towards Ru catalysts.Nevertheless, Ru catalysts are associated with two key challenges: high cost and carbon support methanation in harsh industrial conditions.Consequently, scientists continue to search for improved catalysts.In the process, researchers have found that promising catalysts for ammonia synthesis can be obtained by modifying transition metal elements [35,36].

Transition Metal Catalysts Using Nanomaterials as Carriers
As an example, previous investigations have revealed that cobalt (Co) exhibits a certain degree of nitrogen adsorption ability, and Co/Ce 0.5 Zr 0.5 O 2 , a Co-containing catalyst effectively facilitate N 2 dissolution, making them suitable for ammonia synthesis reactions [37].The catalytic activities of the Co and Co/Ce 0.5 Zr 0.5 O 2 were calculated using a linear combination of fits with the Athena software (1.28.4), with the former showing a catalytic activity of 174.6 µmol g −1 h −1 and the latter showing a catalytic activity of 293.5 µmol g −1 h −1 .These results indicate that the modified Co-containing catalysts have good catalytic activities [38].Although Co is not cheaper than Ru, ammonia can be synthesized at lower temperatures using Co catalysts in the presence of an electric field, thus reducing industrial production costs [37][38][39].Moreover, transition metal-embedded carbon nanotubes (M@NCNTs) possessing the Mott-Schottky feature promoted spontaneous electron transfer between the different domains and elevated the Fermi energy levels of the outer carbon nanotubes and the C-N (π) orbitals formed with N 2 molecules, accelerating the electron transfer from the catalyst to the N 2 molecule and leading to inert N≡N triple bond breakage [30].

Transition Metal Complexes
As early as 1965, Allen and Senoff discovered the transition metal N 2 complex and pioneered the preparation of the first transition metal N 2 complex (Ru(NH 3 ) 5 (N 2 )] 2+ ) [40].Since then, transition metal complexes have shown high value in the academic field.However, there have been few reports until 2003 on the reduction of nitrogen to ammonia catalyzed by transition metal complexes, when Yandulov and Schrock successfully synthesized the chemistry of Mo complexes that contain a triamidoamine (((ArNCH 2 CH 2 ) 3 N) 3-, which is (ArN 3 N) 3− , where Ar is aryl) ligand.Experimental results demonstrated that the most successful efficiency of this class of chemicals in catalyzing the reduction of nitrogen to ammonia (63% to 66%) is second only to that of the highest known catalytic efficiency of Fe/Mo nitrogenase (75%) [41].Until today, transition metal complexes catalyzing NRR have been the focus of research, such as Rb 2 (Mn(NH 2 ) 4 ), K 2 (Mn(NH 2 ) 4 ), and transition metal-LiH composite catalysts, which have been shown to effectively break the scaling relations and achieve ammonia synthesis under mild conditions.Because they exhibit two temperature-dependent polymorphs, that is, a low-temperature orthorhombic and a high-temperature monoclinic structure [42].
Research on electrocatalytic ammonia synthesis has made significant progress in addressing challenges associated with molecular-catalyst mechanisms, leading to improvements in FE and enabling the full realization of the NRR.However, certain issues remain unresolved, including low generation rates and inadequate material characterization technologies.NRR still has a long way to go [19].

Chemical Bond Breaking and Directional Coupling Mechanism in Electrocatalytic N 2 Reduction Reaction
In general, NRR can be expressed by the reaction formula: The widely accepted pathways for the synthesis of NH 3 through the reduction of N 2 on the catalyst surface can generally be divided into two categories: the dissociation pathway and the association pathway.In the dissociation pathway, first, N≡N is directly cleaved to produce two isolated N atoms on the substrate surface; then, the two isolated N atoms are hydrogenated to produce two NH 3 molecules.The associative pathway can be divided into distal, alternating, and enzymatic mechanisms based on the adsorption configurations of N 2 .(1) The distal pathway: N 2 molecules are adsorbed on the substrate in a terminal configuration.Protonation and reduction occur first at the most distal N atom; after three steps of protonation and reduction, NH 3 is successfully synthesized.(2) The alternation pathway: N 2 is adsorbed on the substrate in a terminated configuration.First, protonation and reduction take place at the furthest N atom; then, these reactions alternate to form the *NH 2 NH 2 intermediate; finally, the *NH 2 NH 2 intermediate is protonated and reduced sequentially, forming and releasing two equivalents of NH 3 .(3) The enzyme pathway: N 2 is adsorbed on the substrate in a lateral configuration.Two of the N atoms bind to the substrate surface; protonation and reduction occur sequentially to form the *NH 2 NH 2 intermediate, which is further protonated to produce NH 3 [32].However, due to the extremely high bond energy of the stable N≡N bond (948 kJ mol −1 ), activating N 2 is very challenging in electrochemical systems.To develop a suitable catalyst with the ability to cleave the N≡N bond and improve the efficiency of electrocatalytic ammonia synthesis, scientists have conducted experimental studies [43,44].

N 2 Conversion Using Perovskite and Non-Thermal Plasma
The studies found that perovskite is particularly suitable for oxidation reactions, including the oxidation of nitrogen oxides and carbon monoxide [45][46][47].Additionally, non-thermal plasma (NTP: a gas comprising numerous high-energy electrons at a low temperature) promotes the activation of N 2 under mild conditions [48].Based on these two findings, scientists have proposed a new strategy to address the challenge of breaking the N≡N bond: using LaFeO 3 as a catalyst, N 2 and O 2 in the air undergo oxidation to form NO x in NTP.Subsequently, NO x species react with H 2 O to generate NO 3 − and NO 2 − .Finally, NH 3 is electrochemically reduced from NO x − on a Cu/CuO catalyst.The presence of the catalyst led to a significant increase in the current density in the plasma region, facilitating the activation and conversion of N 2 and O 2 [49,50].To gain a deeper insight into the role of LaFeO 3 in the atomic and molecular-level reaction process, scientists have employed theoretical calculations to assess the Gibbs free energy spectrum (∆G) at 50 • C and 1 atm (see Figure 5).They discovered that the presence of LaFeO 3 on both oxygen-rich and oxygen-poor catalyst surfaces substantially lowered the energy required to activate N 2 when compared to gaseous free radicals.Thus, the presence of the catalyst surface or the catalyst itself is undoubtedly beneficial to the reaction.This strategy effectively activates N 2 and improves the FE of the NRR, while also significantly reducing energy consumption and mitigating the energy crisis [43].
(ΔG) at 50 °C and 1 atm (see Figure 5).They discovered that the presence of LaFeO3 on both oxygen-rich and oxygen-poor catalyst surfaces substantially lowered the energy required to activate N2 when compared to gaseous free radicals.Thus, the presence of the catalyst surface or the catalyst itself is undoubtedly beneficial to the reaction.This strategy effectively activates N2 and improves the FE of the NRR, while also significantly reducing energy consumption and mitigating the energy crisis [43]. .In this case, MoS2 is used as an example.The research indicates that, on the one hand, MoS2 is a graphene-like layered material with a tunable electronic structure and is abundant in nature [52].On the other hand, the presence of electron-deficient/electron-rich regions is an indispensable property for the catalyst to effectively adsorb N2 and destroy the N≡N bond, respectively [53].If it is possible to introduce a disulfide with a metal atom with a large difference in the electron affinity from the Mo as the central atom to form a strong interaction with the MoS2, it is feasible to create electrophilic and nucleophilic regions through charge modulation.FeS2 and MoS2 possess different Fermi energy levels.When they were integrated into an FeS2/MoS2 heterojunction, the formed interface can stimulate spontaneous charge transfer and then generate a .In this case, MoS 2 is used as an example.The research indicates that, on the one hand, MoS 2 is a graphene-like layered material with a tunable electronic structure and is abundant in nature [52].On the other hand, the presence of electron-deficient/electron-rich regions is an indispensable property for the catalyst to effectively adsorb N 2 and destroy the N≡N bond, respectively [53].If it is possible to introduce a disulfide with a metal atom with a large difference in the electron affinity from the Mo as the central atom to form a strong interaction with the MoS 2 , it is feasible to create electrophilic and nucleophilic regions through charge modulation.FeS 2 and MoS 2 possess different Fermi energy levels.When they were integrated into an FeS 2 /MoS 2 heterojunction, the formed interface can stimulate spontaneous charge transfer and then generate a special space-charge region to drive the targeted surface reaction.The band bending at the interface facilitated the charge redistribution, until the electrocatalytic system reached a thermal equilibrium state.Then, the oppositely charged regions arise at the heterointerface and bring about the alteration of the electron density around the interface, which was favorable for the targeted adsorption and activation of inert N 2 molecules.This approach aims to enhance the effective adsorption of N 2 and promote the breaking of N≡N bonds [51].Additionally, some cases such as MoS 2 , SnS 2 , and CoS 2 were also elucidated in detail to reveal conformational relationships.Due to cobalt's lower electron affinity compared to molybdenum, the electrons are transferred from CoS 2 to MoS 2 during bonding.This results in an electron-deficient region near the CoS 2 side, which can accept the lone pair electrons of the N 2 molecules, aiding in the N 2 absorption [54,55].Conversely, a nucleophilic region forms near the MoS 2 side, accumulating a significant charge that can provide electrons to the empty antibonding orbital of N 2 and facilitate the cleavage of the N≡N bonds [56].To test this hypothesis, scientists fabricated nanocomposites of CoS 2 /MoS 2 , which exhibited excellent catalytic activity against the NRR, as anticipated.Following the experiment, the researchers conducted density functional theory (DFT) calculations (Figure 6) to further investigate the catalytic mechanism, with the results once again confirming the hypothesis [57].
to reveal conformational relationships.Due to cobalt's lower electron affinity compared to molybdenum, the electrons are transferred from CoS2 to MoS2 during bonding.This results in an electron-deficient region near the CoS2 side, which can accept the lone pair electrons of the N2 molecules, aiding in the N2 absorption [54,55].Conversely, a nucleophilic region forms near the MoS2 side, accumulating a significant charge that can provide electrons to the empty antibonding orbital of N2 and facilitate the cleavage of the N≡N bonds [56].To test this hypothesis, scientists fabricated nanocomposites of CoS2/MoS2, which exhibited excellent catalytic activity against the NRR, as anticipated.Following the experiment, the researchers conducted density functional theory (DFT) calculations (Figure 6) to further investigate the catalytic mechanism, with the results once again confirming the hypothesis [57].Before officially undertaking the experiment on the electrocatalytic nitrogen synthesis of ammonia reaction, the scientists initially compared the vibration modes of the CoS2/MoS2 nanocomposites with pure MoS2 and pure CoS2, respectively.They discovered that the nanocomposite caused the Mo-S bond to soften and reduced the vibration frequency of Mo-S, thus confirming the strong interaction between CoS2 and MoS2 [58,59].Subsequently, using the CoS2/MoS2 nanocomposites as catalysts for the NRR, the scientists conducted experiments.The experimental data demonstrated that the charge transfer resistance of CoS2/MoS2 (2.9 Ω) was slightly smaller than that of CoS2 (5.6 Ω), but significantly smaller than that of MoS2 (177.4Ω), suggesting that the interface between CoS2 and MoS2 effectively regulated the electronic structure, accelerated the electron transfer process, and promoted the reaction kinetics.Additionally, the double-layer capacitance of CoS2/MoS2 (26.8 mF cm −2 ) was substantially higher than that of CoS2 (1.9 mF cm −2 ) and MoS2 (2.8 mF cm −2 ), indicating that the introduction of CoS2 provided more electrochemically active sites.Furthermore, the evident decrease in the activation energy for the N2 reaction on the Gibbs free energy diagram (Figure 6) indicated that the CoS2/MoS2 nanocomposite exhibited superior catalytic performance for the NRR compared to pure CoS2 and MoS2.Notably, no by-products such as N2H4 were detected during the experiment, further confirming that the CoS2/MoS2 catalyst displayed high selectivity for NH3 formation, which was the objective of developing the new catalyst [52,57,60,61].Before officially undertaking the experiment on the electrocatalytic nitrogen synthesis of ammonia reaction, the scientists initially compared the vibration modes of the CoS 2 /MoS 2 nanocomposites with pure MoS 2 and pure CoS 2 , respectively.They discovered that the nanocomposite caused the Mo-S bond to soften and reduced the vibration frequency of Mo-S, thus confirming the strong interaction between CoS 2 and MoS 2 [58,59].Subsequently, using the CoS 2 /MoS 2 nanocomposites as catalysts for the NRR, the scientists conducted experiments.The experimental data demonstrated that the charge transfer resistance of CoS 2 /MoS 2 (2.9 Ω) was slightly smaller than that of CoS 2 (5.6 Ω), but significantly smaller than that of MoS 2 (177.4Ω), suggesting that the interface between CoS 2 and MoS 2 effectively regulated the electronic structure, accelerated the electron transfer process, and promoted the reaction kinetics.Additionally, the double-layer capacitance of CoS 2 /MoS 2 (26.8 mF cm −2 ) was substantially higher than that of CoS 2 (1.9 mF cm −2 ) and MoS 2 (2.8 mF cm −2 ), indicating that the introduction of CoS 2 provided more electrochemically active sites.Furthermore, the evident decrease in the activation energy for the N 2 reaction on the Gibbs free energy diagram (Figure 6) indicated that the CoS 2 /MoS 2 nanocomposite exhibited superior catalytic performance for the NRR compared to pure CoS 2 and MoS 2 .Notably, no by-products such as N 2 H 4 were detected during the experiment, further confirming that the CoS 2 /MoS 2 catalyst displayed high selectivity for NH 3 formation, which was the objective of developing the new catalyst [52,57,60,61].

Boron, Carbon, and Nitrogen Cooperate with the Nanotube Single Atom during Electrocatalytic Reduction of Nitrogen to Ammonia
At present, single-atom catalysts (SACs) have demonstrated significant advantages in various catalytic reactions.Compared to traditional catalysts, SACs can greatly increase specific activity and reduce noble metal loading.Previous studies have shown that by manipulating the electronic structure of SACs, the catalytic activity and selectivity can be modified [62][63][64][65].
There are two known configurations of N 2 surface adsorption: end-oriented and side-oriented.Experimental results indicate that the side-oriented mode is more resilient to N-N bond elongation.On the other hand, the end-oriented mode exhibits greater adsorbed energy.Consequently, the end-oriented mode is more favorable for stable adsorption and complete activation of N 2 .In this study, structurally optimized Mn is embedded in boron-carbon-nitrogen nanotubes (BCN NTs), enabling the Mn atom to bind with the three surrounding N atoms and securely adsorb to the substrate surface [63,66].In the end-oriented mode, the Mn-N bond measures 1.82 Å, whereas in the side-oriented mode, two Mn-N bonds occur with lengths of 1.88 Å and 1.96 Å, respectively.Postbinding, the N-N bond extends to 1.14 Å (in the terminal mode) and 1.18 Å (in the lateral mode) from its initial 1.12 Å length, indicating a tendency for N≡N to break and favor the formation of NH 3 (see Figure 7) [67].In simpler terms, when N 2 is adsorbed onto the Mn atom in the terminal mode, the N≡N bond length increases, making it easier to break.Additionally, the hybridization of the d orbital of the transition metal (TM) with the antibond π* of N 2 can create a low-energy system, significantly reducing the energy barrier for the reaction.Consequently, MN-embedded BCN NTs have been further validated as a promising catalytic material.
oriented.Experimental results indicate that the side-oriented mode is more resilient to N-N bond elongation.On the other hand, the end-oriented mode exhibits greater adsorbed energy.Consequently, the end-oriented mode is more favorable for stable adsorption and complete activation of N2.In this study, structurally optimized Mn is embedded in boroncarbon-nitrogen nanotubes (BCN NTs), enabling the Mn atom to bind with the three surrounding N atoms and securely adsorb to the substrate surface [63,66].In the end-oriented mode, the Mn-N bond measures 1.82 Å, whereas in the side-oriented mode, two Mn-N bonds occur with lengths of 1.88 Å and 1.96 Å, respectively.Post-binding, the N-N bond extends to 1.14 Å (in the terminal mode) and 1.18 Å (in the lateral mode) from its initial 1.12 Å length, indicating a tendency for N≡N to break and favor the formation of NH3 (see Figure 7) [67].In simpler terms, when N2 is adsorbed onto the Mn atom in the terminal mode, the N≡N bond length increases, making it easier to break.Additionally, the hybridization of the d orbital of the transition metal (TM) with the antibond π* of N2 can create a low-energy system, significantly reducing the energy barrier for the reaction.Consequently, MN-embedded BCN NTs have been further validated as a promising catalytic material.

Mechanism of Molecular-Catalyst Interaction for Electrocatalytic NO3 − Reduction Reaction
During the process of NRR, it has been observed by scientists that the fracture of N≡N requires an extremely high amount of energy, and the solubility of N2 in water is quite low.As a result, the NRR typically exhibits low reaction rates and FE.In order to address this issue, scientists are actively searching for alternative nitrogen sources that possess better properties compared to N2 for electrocatalytic ammonia synthesis.One nitrogen source that has garnered significant attention from researchers is NO3 − .This is primarily due to its high solubility in water and the low bond energy of N-O (requiring only 21.7% of the energy needed for the dissociation of N≡N) [43].Furthermore, the electrocatalysis of NO3 − ammonia synthesis offers a potential solution to the problem of water pollution caused by the use of nitrogenous fertilizers and the discharge of industrial wastewater.

Mechanism of Molecular-Catalyst Interaction for Electrocatalytic NO 3 − Reduction Reaction
During the process of NRR, it has been observed by scientists that the fracture of N≡N requires an extremely high amount of energy, and the solubility of N 2 in water is quite low.As a result, the NRR typically exhibits low reaction rates and FE.In order to address this issue, scientists are actively searching for alternative nitrogen sources that possess better properties compared to N 2 for electrocatalytic ammonia synthesis.One nitrogen source that has garnered significant attention from researchers is NO 3 − .This is primarily due to its high solubility in water and the low bond energy of N-O (requiring only 21.7% of the energy needed for the dissociation of N≡N) [43].Furthermore, the electrocatalysis of NO 3 − ammonia synthesis offers a potential solution to the problem of water pollution caused by the use of nitrogenous fertilizers and the discharge of industrial wastewater.Additionally, the analysis of the thermodynamics and kinetics of this reaction has revealed its potential to reduce energy consumption and alleviate the energy crisis to some extent [68].
The NO 3 − RR starts with the adsorption of NO 3 − on the catalyst surface, which is then reduced to the intermediate product, NO 2 .This NO 2 absorbs more charge and, subsequently, decomposes into NO and N. Through a series of continuous deoxidation and hydrogenation reactions, OH − is produced in water, and NH 4 + and NH 3 are formed.The overall reaction can be summarized as follows: NO 3 − + 6H 2 O + 8e − → NH 3 + 9OH − .It is worth noting that the intermediate NO 2 plays a crucial role in the electrochemical reduction of NO 3 − to ammonia.However, its N-O bond energy is relatively large and difficult to break, making it a limiting step in the overall reaction rate when catalyzed by certain catalysts.Subsequently, NO 2 absorbs more charge and breaks down into NO and N. Nevertheless, the intermediate products NO and N 2 O may detach and form by-products, such as NO and N 2 O, along with several other possible reaction pathways (NO 2 , NO 2 − , N 2 , NH 2 OH, NH 3 , and N 2 H 4 ), thereby significantly reducing the selectivity of NO 3 − RR.It is evident that the hydrogen evolution reaction resulting from the H* combination is the most prevalent competing reaction for NO 3 − RR [69][70][71].By carefully selecting an appropriate catalyst, altering the structural of the catalyst, or developing a new catalyst that allows for specific NO 3 − adsorption, it becomes possible to enhance the selectivity of NO 3 − RR, reduce the production of other undesired products, and improve the reaction activity.Scientists have made considerable progress in developing various types of catalysts, and this section will provide a summary of the four most significant ones: precious metal catalysts, non-precious metal catalysts, metal oxide catalysts, and metal-free catalysts [72].

Noble Metal Catalyst
Noble metals like Au, Pd, Pt, and Ru exhibit excellent catalytic activity for the NO 3 − RR reaction.However, considering the high cost of noble metals, it is important to optimize the atomic utilization.As a result, the most widely studied catalysts for noble metals are monatomic species, nanostructures, and alloys [73,74].Among noble metals, ruthenium (Ru) is commonly used for the electrocatalytic reduction of nitrate to ammonia.For instance, Li et al. developed a strained Ru nanocluster catalyst that efficiently reduces nitrate to ammonia at room temperature, exhibiting rapid kinetics, high selectivity, and strong current density.The catalyst's high-level performance is attributed to the presence of the tensile lattice strain, which enhances the H-H coupling barrier and suppresses the hydrogen evolution reaction (HER), while also facilitating the production of NH 3 through efficient H* generation.Consequently, the strain nanostructures demonstrate exceptional ammonia production rates and maintain high selectivity over a broad range of operational potentials [25,[75][76][77][78].However, due to the characteristics of noble metals, such as their scarcity and high cost, their practical implementation is greatly limited.In contrast, nonnoble metals are abundant and cost effective, which makes them easier to put into practice in production processes.

Non-Noble Metal Catalysts
Non-noble metal catalysts have high selectivity for the NO 3 − RR reaction and can synthesize ammonia cheaply, efficiently, and sustainably.Researchers have extensively investigated highly efficient non-noble metal catalysts for the electrochemical NO 3 − RR reaction [79].The following discussion focuses on examples using Cu and Fe catalysts [80].
Cu-Based Electrocatalyst Reacts in the Nitrate Reduction Reaction (NO x − RR) Metallic Cu-based materials are the most widely studied electrocatalysts for the NO 3 − RR reaction due to their favorable NO 3 − adsorption, high FE at a low current density, excellent selectivity, effective HER inhibition, and low cost.The Cu catalyst undergoes restructuring during NO 3 − RR electrocatalysis, with the extent depending on the Cu loading and reaction potential.Under negative reaction potentials, Cu monoatomic sites readily aggregate, transforming into Cu clusters and nanoparticles.These reconstructed Cu species can reversibly transform back to Cu monoatoms through peroxidation-driven redispersion under environmental conditions.The reconstructed form of copper significantly enhances the rate of ammonia reduction.For instance, experiments have demonstrated the effectiveness of the copper-modified covalent triazine skeleton (Cu-CTF) as an electrocatalyst for the reduction of nitrate, as depicted in Figure 8 [81].Nevertheless, the cumulative impact of nitrite (NO 2 − ) on copper, along with the recombination of highly active Cu-based electrocatalyst during the NO 3 − RR reaction, makes it challenging to identify the dynamic active sites and conduct comprehensive studies on the catalytic mechanism.Consequently, selective NH 3 production using copper as an electrocatalyst remains unadvisable [82][83][84][85][86]. Compared with other non-noble metal catalysts, Cu-based electrocatalysts are more easily used in actual production.
cumulative impact nitrite (NO2 − ) on copper, along with the recombination of highly active Cu-based electrocatalyst during the NO3 − RR reaction, makes it challenging to identify the dynamic active sites and conduct comprehensive studies on the catalytic mechanism.Consequently, selective NH3 production using copper as an electrocatalyst remains unadvisable [82][83][84][85][86]. Compared with other non-noble metal catalysts, Cu-based electrocatalysts are more easily used in actual production.Fe-Based Electrocatalysts for NO3 − RR Fe-based monatomic catalysts are synthesized using a TM-assisted carbonization method and SiO2 powder as a hard template.The catalytic performance of these catalysts in electrochemical NO3 − RR is primarily dependent on the activity of iron active sites and the number of available active sites.Nitrogen is uniformly dispersed in order to coordinate the positive charge introduced by the metal sites, and it has a moderate interaction with NO3 − , resulting in excellent NO3 − RR performance.Analysis of the X-ray spectra reveals that the oxidation state of Fe single atoms lies between Fe 2+ and Fe 3+ .Delocalized electrons of Fe single atoms can be shared by porphyrin-like structures, which enhances the catalyst's conductivity while reducing its nitrate catalytic activity.Experiments conducted by Wu indicate that the concentration of NO3 − has no significant effect on the selectivity of NH3 on Fe-based monatomic catalysts.However, the content of NO2 − generation decreases with increasing iron-based monatomic catalysts and the electric current, thereby leading to an increase in NH3 selectivity (as shown in Figure 9) [87][88][89][90][91][92].However, due to the high uncertainty of the active site, further research is still needed.Fe-Based Electrocatalysts for NO 3 − RR Fe-based monatomic catalysts are synthesized using a TM-assisted carbonization method and SiO 2 powder as a hard template.The catalytic performance of these catalysts in electrochemical NO 3 − RR is primarily dependent on the activity of iron active sites and the number of available active sites.Nitrogen is uniformly dispersed in order to coordinate the positive charge introduced by the metal sites, and it has a moderate interaction with NO 3 − , resulting in excellent NO 3 − RR performance.Analysis of the X-ray spectra reveals that the oxidation state of Fe single atoms lies between Fe 2+ and Fe 3+ .Delocalized electrons of Fe single atoms can be shared by porphyrin-like structures, which enhances the catalyst's conductivity while reducing its nitrate catalytic activity.Experiments conducted by Wu indicate that the concentration of NO 3 − has no significant effect on the selectivity of NH 3 on Fe-based monatomic catalysts.However, the content of NO 2 − generation decreases with increasing iron-based monatomic catalysts and the electric current, thereby leading to an increase in NH 3 selectivity (as shown in Figure 9) [87][88][89][90][91][92].However, due to the high uncertainty of the active site, further research is still needed.

Metal Oxide Catalyst
Transition metal oxides have several advantages, such as natural abundance, ecological friendliness, and chemical stability, and are suitable as catalysts to catalyze reactions.Among them, Cu, Co, Ag, Ti, and Ru, primarily form oxides.These oxides exhibit various structures and phases, including defects and oxygen vacancies.These oxides effectively catalyze nitrate ions to ammonia, exhibiting outstanding nitrate conversion efficiency, a significant ammonia yield or conversion, high FE, and strong ammonia selectivity [93,94].One example is the CuO catalyst, composed of copper oxide (CuO).CuO nanowire arrays exhibit an impressive FE (95.8%) and high ammonia selectivity (81.2%).They serve as highly efficient cathode materials for electrocatalyzing the reduction of nitrate ions to ammonia [84,95,96].Another example is iron oxide, including Fe 2 O 3 and Fe 3 O 4 .Activation of the sample with Fe 2 O 3 single-bond carbon nanotubes resulted in a substantial decrease in the current density.However, the NH 3 generation rate remained high, suggesting a change in the catalyst's performance during activation.This change not only suppressed HER competition, but also enhanced the FE of NH 3 (Figure 10) [97].In summary, transition metal oxide catalysts have excellent stability, but there are still some difficulties in practical implementation.

Metal Oxide Catalyst
Transition metal oxides have several advantages, such as natural abundance, ecological friendliness, and chemical stability, and are suitable as catalysts to catalyze reactions.Among them, Cu, Co, Ag, Ti, and Ru, primarily form oxides.These oxides exhibit various structures and phases, including defects and oxygen vacancies.These oxides effectively catalyze nitrate ions to ammonia, exhibiting outstanding nitrate conversion efficiency, a significant ammonia yield or conversion, high FE, and strong ammonia selectivity [93,94].One example is the CuO catalyst, composed of copper oxide (CuO).CuO nanowire arrays exhibit an impressive FE (95.8%) and high ammonia selectivity (81.2%).They serve as highly efficient cathode materials for electrocatalyzing the reduction of nitrate ions to ammonia [84,95,96].Another example is iron oxide, including Fe2O3 and Fe3O4.Activation of the sample with Fe2O3 single-bond carbon nanotubes resulted in a substantial decrease in the current density.However, the NH3 generation rate remained high, suggesting a change in the catalyst's performance during activation.This change not only suppressed HER competition, but also enhanced the FE of NH3 (Figure 10) [97].In summary, transition metal oxide catalysts have excellent stability, but there are still some difficulties in practical implementation.

Metal-Free Catalyst
Compared to metal catalysts, metal-free catalysts offer several advantages, including lower costs, greater environmental friendliness, and reduced toxicity.In summary, the development and exploration of metal-free catalysts holds significant scientific importance and economic value.Currently, carbon-based catalysts are predominantly reported as electrochemical NRR metal-free catalysts.These catalysts function by disrupting the homogeneous charge density of carbon atoms through the introduction of foreign atoms with different electronegativity and atomic radius.This process leads to the formation of more free electrons in the delocalized π orbital of the carbon skeleton, thereby enhancing the catalytic activity [91,[98][99][100][101].
To exemplify the impact of a metal-free catalyst on the electrocatalytic reduction of ammonia with nitrate, we will analyze the reduction of graphene oxide.Wang and his team investigated the electrocatalytic reduction of nitrate to ammonia mechanism (NRA) of PdP 2 nanoparticles on reduced graphene oxide (PdP 2 /RGO) [102].Initially, they examined the adsorption of three NO 3 − geometries on the PdP 2 surface: top Pd site 1, top Pd site 2, and bridge Pd site.Comparative analysis of the adsorption free energy of NO 3 − at different sites revealed a preference for NO 3 − absorption at the bridge Pd site (Figure 11) [102].Furthermore, the analysis of the partial density of the state indicated significant hybridization between NO 3 − p and Pd d orbitals, demonstrating a strong interaction between Pd and *NO 3 − .This analysis suggests that the PdP 2 surface effectively activates and hydrogenates NO 3 − , while inhibiting the competing HER [103].

Metal-Free Catalyst
Compared to metal catalysts, metal-free catalysts offer several advantages, including lower costs, greater environmental friendliness, and reduced toxicity.In summary, the development and exploration of metal-free catalysts holds significant scientific importance and economic value.Currently, carbon-based catalysts are predominantly reported as electrochemical NRR metal-free catalysts.These catalysts function by disrupting the homogeneous charge density of carbon atoms through the introduction of foreign at- 2, and bridge site.Comparative analysis of the adsorption free energy of NO3 − at different sites revealed a preference for NO3 − absorption at the bridge Pd site (Figure 11) [102].Furthermore, the analysis of the partial density of the state indicated significant hybridization between NO3 − p and Pd d orbitals, demonstrating a strong interaction between Pd and *NO3 − .This analysis suggests that the PdP2 surface effectively activates and hydrogenates NO3 − , while inhibiting the competing HER [103].

Chemical Bond Breaking and Directional Coupling Mechanism in Electrocatalytic NO3 − Reduction Reaction
The electrocatalytic NO3 − reduction reaction involves eight electron transfers and nine proton transfers, presenting a significant challenge for reaction selectivity [68].As an illustration, the selectivity between nitrite and ammonia gas is examined to explain the directional coupling mechanism in the electrocatalytic ammonia synthesis from nitrate.

Reduction Reaction
The electrocatalytic NO 3 − reduction reaction involves eight electron transfers and nine proton transfers, presenting a significant challenge for reaction selectivity [68].As an illustration, the selectivity between nitrite and ammonia gas is examined to explain the directional coupling mechanism in the electrocatalytic ammonia synthesis from nitrate.Incomplete conversion of nitrates can result in the formation of nitrite (NO 2 − ), which can cause liver damage, hyperhemoglobinemia, and potential cancer development in humans.The formation of HNO 2 is facilitated by overlapping the reaction steps with NH 3 production.Analysis of the kinetic barriers reveals that the protonation of NO 2 to HNO 2 is crucial for determining product selectivity.At −0.50 V vs. RHE, the protonation of NO 2 * to HNO 2 exhibits a lower potential barrier (0.48 eV) compared to the highest barrier for NH 3 formation (0.55 eV) (Figure 12).Subsequent investigations involving microdynamics modeling, electronic structure analysis, and consistent energy barrier changes, provided further evidence that the difference in the protonation barriers between NO 2 and HNO 2 or cisHNO 2 is crucial for determining product selectivity.Moreover, due to the smaller charge transfer coefficient for the protonation of NO 2 * to HNO 2 at higher overpotentials, the production of HNO 2 is favored.Existing experimental data indicates that FeN 4 exhibits superior catalytic performance compared to other catalysts, including CuN 4 , NiN 4 , and CoN 4 , and promotes the NH 3 generation pathway.Moreover, NO 3 − RR produces NH 3 via the pathway on 4 (R4: NO 2 →cisHNO 2 ) and HNO 2 generation (R7: NO 2 →HNO 2 ) is nearly equal, whereas the free energy of HNO 2 formation (R7: NO 2 →HNO 2 ) for the other three catalysts (CuN 4 , NiN 4 , CoN 4 ) is lower than that of the NH 3 selection step (R10: NO→HNO), highlighting a preference for HNO 2 production.The discovery of FeN 4 offers valuable insights for the development of novel and enhanced catalysts, prompting researchers to conduct further experimentation and design catalysts that align with this concept [104].
ing, electronic structure analysis, and consistent energy barrier changes, provided further evidence that the difference in the protonation barriers between NO2 and HNO2 or cisHNO2 is crucial for determining product selectivity.Moreover, due to the smaller charge transfer coefficient for the protonation of NO2* to HNO2 at higher overpotentials, the production of HNO2 is favored.Existing experimental data indicates that FeN4 exhibits superior catalytic performance compared to other catalysts, including CuN4, NiN4, and CoN4, and promotes the NH3 generation pathway.Moreover, NO3 − RR produces NH3 via the pathway on FeN4 (R4: NO2→cisHNO2) and HNO2 generation (R7: NO2→HNO2) is nearly equal, whereas the free energy of HNO2 formation (R7: NO2→HNO2) for the other three catalysts (CuN4, NiN4, CoN4) is lower than that of the NH3 selection step (R10: NO→HNO), highlighting a preference for HNO2 production.The discovery of FeN4 offers valuable insights for the development of novel and enhanced catalysts, prompting researchers to conduct further experimentation and design catalysts that align with this concept [104].

Conclusions and Perspectives
In order to help readers understand the advantages of the various catalysts listed in this review, some examples of the different types of catalysts for the two reactants mentioned in the text have been compiled in this review, which aim to show readers in a clear and concise manner the effects of different catalysts on ammonia yield and FE in the electrocatalytic ammonia synthesis reaction (Tables 1 and 2).

Conclusions and Perspectives
In order to help readers understand the advantages of the various catalysts listed in this review, some examples of the different types of catalysts for the two reactants mentioned in the text have been compiled in this review, which aim to show readers in a clear and concise manner the effects of different catalysts on ammonia yield and FE in the electrocatalytic ammonia synthesis reaction (Tables 1 and 2).
Researchers have extensively explored various methods and developed numerous catalysts to address issues related to low selectivity for NH 3 and high energy consumption during the NRR and NO 3 − RR processes.In existing studies, transition metals have always played an important role in ammonia synthesis.Nonetheless, challenges persist, including the high cost of catalysts and the inability to completely eliminate competitive reactions.In 2018, scientists used the concept of main-group metal mimetization to speculate that the NRR can be made efficient under transition metal-free conditions.Materials such as carbene, di-coordinated boron olefins, and others were later shown to be able to cleave the N≡N bond by repeating reduction-protonation steps from an end-on bridging N 2 complex, successfully proving the scientists' conjecture, and in the future, non-transition-metal catalysts may also become an emerging research hotspot [105].

Figure 1 .
Figure 1.The main elements of electrocatalytic ammonia synthesis discussed in this review.

Figure 2 .
Figure 2. Diagram of thermal catalysis, electrocatalysis, photocatalysis, and chemical cyclization processes for the synthesis of NH3 from renewable energy sources.Copyright 2019, Elsevier.

Figure 1 .
Figure 1.The main elements of electrocatalytic ammonia synthesis discussed in this review.

Figure 1 .
Figure 1.The main elements of electrocatalytic ammonia synthesis discussed in this review.

Figure 2 .
Figure 2. Diagram of thermal catalysis, electrocatalysis, photocatalysis, and chemical cyclization processes for the synthesis of NH3 from renewable energy sources.Copyright 2019, Elsevier.

Figure 2 .
Figure 2. Diagram of thermal catalysis, electrocatalysis, photocatalysis, and chemical cyclization processes for the synthesis of NH 3 from renewable energy sources.Copyright 2019, Elsevier.

Figure 3 .
Figure 3. Illustration of the use of renewable electricity to drive electrocatalytic nitrogen synthesis of ammonia, and important applications of ammonia: carbon-free fuels (future) and nitrogen fertilizers.Copyright 2020, Elsevier.

Figure 3 .
Figure 3. Illustration of the use of renewable electricity to drive electrocatalytic nitrogen synthesis of ammonia, and important applications of ammonia: carbon-free fuels (future) and nitrogen fertilizers.Copyright 2020, Elsevier.

Figure 4 .Figure 4 .
Figure 4. (a) FE and NH3 yield rate for NiTe-800 within the potential, ranging from −0.1 to 0.5 V. (b) LSV curves for NiTe-800.(c) The chronoamperometric curves for NiTe-800 at various potentials for Figure 4. (a) FE and NH 3 yield rate for NiTe-800 within the potential, ranging from −0.1 to 0.5 V. (b) LSV curves for NiTe-800.(c) The chronoamperometric curves for NiTe-800 at various potentials for 2 h in HCl solution.(d) FE and NH 3 yield rate for NiTe-700 and NiTe-800.(e) The chronoamperometric curves for NiTe-800 for 24 h in HCl solution.(f) FE and NH 3 yield rate for NiTe-800 at −0.1 V during recycling tests repeated five times.Copyright 2020, John Wiley and Sons.

Figure 5 .
Figure 5. Gibbs free energy profile of N2 reaction on the different surfaces at 50 °C and 1 atm.* represent catalytic active sites.Copyright 2022, American Chemical Society.2.2.2.Modification of Mo-Based Catalysts to Promote N2 Adsorption for Improved NH3 Yield and FE Drawing inspiration from biological nitrogen fixation, scientists have developed various catalysts based on transition metal elements as the primary active centers, resembling the function of nitrogenase [51].In this case, MoS2 is used as an example.The research indicates that, on the one hand, MoS2 is a graphene-like layered material with a tunable electronic structure and is abundant in nature[52].On the other hand, the presence of electron-deficient/electron-rich regions is an indispensable property for the catalyst to effectively adsorb N2 and destroy the N≡N bond, respectively[53].If it is possible to introduce a disulfide with a metal atom with a large difference in the electron affinity from the Mo as the central atom to form a strong interaction with the MoS2, it is feasible to create electrophilic and nucleophilic regions through charge modulation.FeS2 and MoS2 possess different Fermi energy levels.When they were integrated into an FeS2/MoS2 heterojunction, the formed interface can stimulate spontaneous charge transfer and then generate a

Figure 5 .
Figure 5. Gibbs free energy profile of N 2 reaction on the different surfaces at 50 • C and 1 atm.* represent catalytic active sites.Copyright 2022, American Chemical Society.2.2.2.Modification of Mo-Based Catalysts to Promote N 2 Adsorption for Improved NH 3 Yield and FE Drawing inspiration from biological nitrogen fixation, scientists have developed various catalysts based on transition metal elements as the primary active centers, resembling the function of nitrogenase [51].In this case, MoS 2 is used as an example.The research indicates that, on the one hand, MoS 2 is a graphene-like layered material with a tunable electronic structure and is abundant in nature[52].On the other hand, the presence of electron-deficient/electron-rich regions is an indispensable property for the catalyst to effectively adsorb N 2 and destroy the N≡N bond, respectively[53].If it is possible to introduce a disulfide with a metal atom with a large difference in the electron affinity from the Mo as the central atom to form a strong interaction with the MoS 2 , it is feasible to create electrophilic and nucleophilic regions through charge modulation.FeS 2 and MoS 2 possess different Fermi energy levels.When they were integrated into an FeS 2 /MoS 2 heterojunction, the formed interface can stimulate spontaneous charge transfer and then generate a special space-charge region to drive the targeted surface reaction.The band bending at the interface facilitated the charge redistribution, until the electrocatalytic system reached a thermal equilibrium state.Then, the oppositely charged regions arise at the heterointerface and bring about the alteration of the electron density around the interface, which was favorable for the targeted adsorption and activation of inert N 2 molecules.This approach aims to enhance the effective adsorption of N 2 and promote the breaking of N≡N bonds[51].Additionally, some cases such as MoS 2 , SnS 2 , and CoS 2 were also elucidated in detail to reveal conformational relationships.Due to cobalt's lower electron affinity compared to molybdenum, the electrons are transferred from CoS 2 to MoS 2 during bonding.This results in an electron-deficient region near the CoS 2 side, which can accept the lone pair electrons of the N 2 molecules, aiding in the N 2 absorption[54,55].Conversely, a nucleophilic region forms near the MoS 2 side, accumulating a significant charge that can provide electrons to the empty antibonding orbital of N 2 and facilitate the cleavage of the N≡N bonds[56].To test this hypothesis, scientists fabricated nanocomposites of

Figure 7 .
Figure 7.The relaxed configuration of N2 in the (a) end-orientated mode; (b) side-orientated mode on a Mn-embedded BCN NT.Black, yellow, violet and green color represent the C, B, N and Mn stoms.Copyright 2022, Elsevier.

Figure 7 .
Figure 7.The relaxed configuration of N 2 in the (a) end-orientated mode; (b) side-orientated mode on a Mn-embedded BCN NT.Black, yellow, violet and green color represent the C, B, N and Mn stoms.Copyright 2022, Elsevier.

Figure 8 .
Figure 8.The path diagram of the copper-modified covalent triazine skeleton (Cu-CTF) catalyzed nitrate electrochemical reduction and the results of the Cu-CTF density functional calculation.Copyright 2016, American Chemical Society.

Figure 8 .
Figure 8.The path diagram of the copper-modified covalent triazine skeleton (Cu-CTF) catalyzed nitrate electrochemical reduction and the results of the Cu-CTF density functional calculation.Copyright 2016, American Chemical Society.

Figure 9 .
Figure 9. (a) Linear sweep voltammetry curves for the Fe SAC in K2SO4 electrolyte and KNO3/K2SO4 mixed electrolyte.(b) NH3 FE for Fe SAC at each given potential.Red dot is FE estimated by three independent NMR tests.(c) NH3 yield rate and partial current density for Fe SAC, FeNP/NC, and NC.(d) Nuclear magnetic resonance hydrogen spectra for the electrolytes after three independent nitrate reduction tests.(e) Nuclear magnetic resonance hydrogen spectra of electrolyte after 15 NO3 − reduction test with K 15 NO3/K2SO4 mixed electrolyte at different times.(f) NH3 yield rate for Fe SAC, Co SAC, and Ni SAC based on metal content.(g) The cycling tests for Fe SAC.Catalyst loading for all of the electrocatalytic nitrate reduction tests is the same.Copyright 2021, Nature.

Figure 9 .
Figure 9. (a) Linear sweep voltammetry curves for the Fe SAC in K 2 SO 4 electrolyte and KNO 3 /K 2 SO 4 mixed electrolyte.(b) NH 3 FE for Fe SAC at each given potential.Red dot is FE estimated by three independent NMR tests.(c) NH 3 yield rate and partial current density for Fe SAC, FeNP/NC, and NC.(d) Nuclear magnetic resonance hydrogen spectra for the electrolytes after three independent nitrate reduction tests.(e) Nuclear magnetic resonance hydrogen spectra of electrolyte after 15 NO 3 − reduction test with K 15 NO 3 /K 2 SO 4 mixed electrolyte at different times.(f) NH 3 yield rate for Fe SAC, Co SAC, and Ni SAC based on metal content.(g) The cycling tests for Fe SAC.Catalyst loading for all of the electrocatalytic nitrate reduction tests is the same.Copyright 2021, Nature.

Figure 10 .
Figure 10.(a) Sample current density before and after activation.(b) Ammonia formation rate and Faradaic efficiency.Copyright 2020, Elsevier.

Figure 10 .
Figure 10.(a) Sample current density before and after activation.(b) Ammonia formation rate and Faradaic efficiency.Copyright 2020, Elsevier.

Figure 11 .
Figure 11.(a) Configurations of *NO3 on PdP2.(b) Differential charge-density map of *NO3 on PdP2.Yellow and cyan denote charge accumulation and depletion, respectively.* represent the catalytic active sites.(c) Adsorption free energy diagrams of PdP2 and the corresponding intermediates.Copyright 2023, American Chemical Society.

Figure 11 .
Figure 11.(a) Configurations of *NO 3 on PdP 2 .(b) Differential charge-density map of *NO 3 on PdP 2 .Yellow and cyan denote charge accumulation and depletion, respectively.* represent the catalytic active sites.(c) Adsorption free energy diagrams of PdP 2 and the corresponding intermediates.Copyright 2023, American Chemical Society.

Figure 12 .
Figure 12.Kinetic barriers on FeN4 to NH3 and HNO2 at −0.50 V vs. RHE.The pathways to produce NH3 and HNO2 are highlighted with green and red lines, respectively.* represent the catalytic active sites.The pink, red, light blue, brown, and bright yellow represent H, O, N, C, and Fe atoms, respectively.Copyright 2023, Elsevier.

Figure 12 .
Figure 12.Kinetic barriers on FeN 4 to NH 3 and HNO 2 at −0.50 V vs. RHE.The pathways to produce NH 3 and HNO 2 are highlighted with green and red lines, respectively.* represent the catalytic active sites.The pink, red, light blue, brown, and bright yellow represent H, O, N, C, and Fe atoms, respectively.Copyright 2023, Elsevier.

Table 1 .
Summary of electrocatalysts for the reduction of N2 to NH3.

Table 1 .
of electrocatalysts for the reduction of N 2 to NH 3 .

Table 2 .
Summary of electrocatalysts for the reduction of NO 3 − to NH 3 .